Four years ago, the wall came tumbling down at the Justice Department.

Not a literal wall, but the operational one separating intelligence and law enforcement investigations, the archaic structure considered to be one of the causes of the government's failure to prevent the Sept. 11, 2001, attacks.

The department created a National Security Division, combining criminal prosecutors and intelligence lawyers into one unit that President George W. Bush vowed would help "connect the dots before the terrorists strike." A little-noticed, yet key post-Sept. 11 reform, it was the first new Justice Department division created since 1957.

Four years later, justice officials say the division is performing as promised, melding counterintelligence and law enforcement, and helping to prevent attacks. They point to a thwarted plot to bomb New York City's subways last year. More defendants were charged in federal court last year than in any year since 2001 with the most serious types of terrorism violations, amid growing concern about homegrown terrorist plots.

"We have never seen a threat environment like the one we've been facing, since Sept. 11,'' said Assistant Attorney General David S. Kris, who heads the unit. "It has been a crucible for this division, and I think it has resulted in a proof of concept: It turns out it was a good idea to create the National Security Division. It really works.''

Others offered mixed appraisals of the unit, which is based at Justice Department headquarters and has grown from 202 staffers to nearly 350. Advocates for open government have faulted it for aggressively pursuing leaks of classified information. And, under the Obama administration, congressional Republicans have accused the unit of returning to a pre-Sept. 11 mind-set that fails to emphasize intelligence and preventing attacks.

"The National Security Division is a good concept,'' said Sen. Christopher S. Bond (Mo.), the top Republican on the Intelligence Committee. "But I am increasingly concerned that they are returning to a law enforcement approach. They talk a good game, but they treat terrorists as normal criminal defendants.''

Congressional Democrats and former Justice Department officials praise Kris's legal savvy and the unit's aggressive approach to breaking up terrorist plots, and Laurence H. Silberman, a senior judge on the U.S. Court of Appeals for the D.C. Circuit, said it is "doing an excellent job."

"They fully understand the imperatives of the intelligence community and that sometimes prosecution is a useful tool and sometimes it's not,'' Silberman said.

It was a 2005 report on the government's intelligence failures, from a commission co-chaired by Silberman, that led to the new division. In a memorable phrase, the report said the Justice Department's national security structure was "madness.''

For two decades before the Sept. 11 attacks, FBI agents and prosecutors focused on criminal investigations had been segregated from government agents focused on intelligence-gathering against foreign spies and international terrorists - the so-called wall. The structure had grown out of changes in federal law stemming from a string of abuses by government agents in the 1960s and 1970s, including spying on antiwar activists.

With separate teams of intelligence and law enforcement agents sometimes focused on the same terrorist targets - and not sharing information - the wall would be cited as a key reason the government failed to prevent the Sept. 11 attacks. The passage of the USA Patriot Act after the attacks led to numerous reforms at the FBI and other agencies.